The principle whereby such sheets based on carbonized filaments or yarn are activated consists in subjecting such filaments or yarn to oxidation to increase the surface microporosity thereof so as to develop a special surface favorable for retaining molecules. Such a surface thus offers numerous fixing sites which define the retention power and thus the filtering power of a fibrous sheet interposed as a screen for passing through between two different media between which a liquid or gaseous fluid flows in one direction, or the other, or in both directions, conveying polluting molecules that are to be retained.
Such a technique can be considered as known in principle and applicable to woven or non-woven textile sheets based on or totally constituted by continuous carbonized filaments or spun carbonized yarn.
There have been several proposals in the prior art in this field and they can be classified in two families.
The first family corresponds to that which can be referred to as a "discontinuous method", as illustrated, for exemple, by patent GB 1 570 177.
The second family corresponds to a treatment technique that can be referred to as "continuous" and it is illustrated by patent GB 1 310 011 and application EP 0 626 548.
The present invention relates specifically to the activation technique corresponding to the second family.
For continous activation treatment, the above British patent recommends placing vertically-extending heater means face to face in an enclosure leaving between them a passage along which a fibrous sheet travels upwardly and is thus subjected on both faces to the action of the heater elements.
A nozzle for dispensing or feeding an oxidizing gaseous fluid is provided from the bottom of the enclosure so as to fill the passage along which the sheet travels with said fluid.
On the basis of that proposition which is already several tens of years old, it was expected that activation would take place suitable for giving satisfaction in the intended implementation.
In fact, that manner of proceeding seems to be incapable of leading to satisfactory and reproducible results for several reasons.
Firstly, the vertical disposition cannot be considered as providing a uniform distribution of the oxidizing gaseous fluid which is subjected to a turbulent type upward flow mainly because of the heat given off by the heater elements and because of the thermal updraft which is established in the passage.
Thus, the action of such an oxidizing fluid cannot be considered as being performed in sufficiently uniform manner from the bottom to the top of the sheet and the fibers thereof are inevitably subjected to non-negligible temperature differences.
Also, in the recommended disposition, there does not exist any genuine confinement volume for the oxidizing gaseous fluid whose upward flow cannot be properly controlled relative to the upward vertical displacement of the sheet that is to be treated.
According to the English patent, the structural dispositions described seek to dispose the rollers of payout and takeup spools inside the enclosure, thus necessarily giving rise to an access problem during which the enclosure must be opened.
Given the nature of the oxidizing gaseous fluids and of the internal reactions that take place during activation, it is therefore necessary to take considerable precautions prior to opening the enclosure to avoid any leak of gas molecules that could harm the environment or the staff operating such an installation.
Given the general configuration recommended, it is also important in that technique to provide bends in the path so as to cause only that portion of the sheet which extends between the heater elements to travel vertically. Rollers or analogous deflector members are therefore necessary and impart mechanical stresses to the fibers before and after treatment without it being possible to ensure appropriate stabilization of mean temperature, either by progressively raising the temperature before activation or by lowering said temperature after coming out from the heater elements.
Application EP 0 626 548 describes a method and an apparatus for high-speed oxidation of a textile sheet based on organic fibers. According to that application, the textile sheet to be treated passes through a chamber containing an oxidizing atmosphere raised to a temperature lying in the range 200.degree. C. to 300.degree. C. The fibrous sheet to be treated is guided by cooling rollers disposed outside the oxidizing chamber so as to ensure that the sheet passes several times therethrough in succession. Those cooling rollers prevent the temperature of the sheet rising excessively. Finally, according to that application, the sheet travels through the chamber at a linear speed lying in the range 10 meters per minute (m/min) and 50 m/min.
In use, it has turned out that the conditions under which that method is implemented, in particular the conditions relating to travel speed and the temperatures reached, do not enable optimum oxidation of sheets based on carbonized filaments to be obtained.
In addition, the use of cooling rollers for deflection purposes subjects the sheet being treated to stresses that can spoil its mechanical qualities.
Finally, the successive deflections make it necessary to provide a certain number of inlet and outlet zones for the sheet through the walls of the oxidizing chamber, thus constituting as many openings through which polluting gases produced by the oxidation reactions can escape.